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One aim of modern astronomy is to detect temperate, Earth-like exoplanets that are well suited for atmospheric characterization. Recently, three Earth-sized planets were detected that transit (that is, pass in front of) a star with a mass just eight per cent that of the Sun, located 12 parsecs away1. The transiting configuration of these planets, combined with the Jupiter-like size of their host star—named TRAPPIST-1—makes possible in-depth studies of their atmospheric properties with present-day and future astronomical facilities1, 2, 3. Here we report the results of a photometric monitoring campaign of that star from the ground and space. Our observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inwards4, 5. Moreover, the seven planets have equilibrium temperatures low enough to make possible the presence of liquid water on their surfaces6, 7, 8.

Considering the hype NASA tried to build around this discovery, I'm a little let down. I was hoping it'd be unambiguous spectral lines for chlorophyll outside of Earth.

The cartoon above is a rough estimate of what the planets in the TRAPPIST-1 system would look like from the surface of its innermost planet, B. I’ve included the moon as seen from the earth for scale. Keep in mind that the our sun appears essentially the same size as our moon (that’s how you get solar eclipses). Also keep in mind that although the star TRAPPIST-1 would appear to be much larger than our sun we are (a) viewing it from the nearest planet and (b) it gives off most of its light in the infrared spectrum, so it would actually appear quite dim. Huge, but dim.

TRAPPIST-1 is also the most tightly packed collection of planets discovered to date. The outermost planet of HD 10180 has a cool semimajor axis of 3.4 AU, while the outermost planet of Kepler-90 orbits at about 1 AU (the same as the distance of Earth from the Sun). Among the known six-planet systems, the outermost planets of Kepler-11 and Kepler-20 have semimajor axes of 0.46 AU and 0.35 AU, respectively. By contrast, the semimajor axis of the seventh planet (h) of TRAPPIST-1 is only 0.06 AU, similar to the orbit of a Hot Jupiter. In this regard, the system’s nearest rivals are Kepler-444, with five subterrestrial planets orbiting within 0.08 AU, and Kepler-80, with five terrestrial and gas dwarf planets within a similar radius. Both of the latter systems center on K-type stars, demonstrating that ultra-compact architectures are not unique to ultra-cool M dwarfs.

One of the better and more comprehensive articles on TRAPPIST-1, despite the fact they're quoting The Sun.

With the discovery of rocky planets in the temperate habitable zone (HZ) of the close-by cool star TRAPPIST-1 the question of whether such planets could harbour life arises. Habitable planets around red dwarf stars can orbit in radiation environments that can be life-sterilizing. UV flares from these stars are more frequent and intense than solar flares. Additionally, their temperate HZs are closer to the star. Here we present UV surface environment models for TRAPPIST-1's HZ planets and explore the implications for life. TRAPPIST-1 has high X-ray/EUV activity, placing planetary atmospheres at risk from erosion. If a dense Earth-like atmosphere with a protective ozone layer exists on planets in the HZ of TRAPPIST-1, UV surface environments would be similar to present-day Earth. However an eroded or an anoxic atmosphere, would allow more UV to reach the surface, making surface environments hostile even to highly UV-tolerant terrestrial extremophiles. If future observations detect ozone in the atmospheres of any of the planets in the HZ of TRAPPIST-1, these would be interesting targets for the search for surface life. We anticipate our assay to be a starting point for in-depth exploration of stellar and atmospheric observations of the TRAPPIST-1 planets to constrain their UV-surface-habitability.

tl;dr:

Quote:

The UV surface habitability of planets in the habitable zone (HZ) of the TRAPPIST-1 system depends critically on the activity of the star and the planet's atmospheric composition. The stellar UV environment will also influence the detectable atmospheric features, including any detectable biosignatures. The detection of atmospheric ozone would indicate that a planet is more likely to have habitable UV surface environments. Analysi of the originally announced TRAPPIST-1 planets was found to be possible with the James Webb Space Telescope (JWST) which could detect present-day ozone levels after 60 transits of the original innermost planet and 30 transits of original outermost planets. Hence, future searches for ozone, combined with specral observations of the host star's activity in the 100-40 nm range will enable us to constrain the radiation environments of the HZ planets around TRAPPIST-1, and their surface UV habitability.

Now an analysis of data from the Kepler space telescope indicates that Trappist-1 is subject to frequent solar flares, blasting its planets with magnetic storms 100 to 10,000 times stronger than the most intense storms hitting the Earth. The implication is that these would disrupt any atmospheres and make the development of complex life unlikely.

They found that, solely looking at the likelihood of retaining an atmosphere over the long haul, Trappist-1g and 1h may be the most likely planets in the system to be habitable. Furthermore, if the planets have a truly resilient atmosphere that keeps climatic conditions relatively stable over billions of years, it also ups the chances that not just life, but diverse biology and even intelligent life might exist there now or at some point in the future.

Scientists now have a good estimate for the age of one of the most intriguing planetary systems discovered to date -- TRAPPIST-1, a system of seven Earth-size worlds orbiting an ultra-cool dwarf star about 40 light-years away. Researchers say in a new study that the TRAPPIST-1 star is quite old: between 5.4 and 9.8 billion years. This is up to twice as old as our own solar system, which formed some 4.5 billion years ago.

Observations of TRAPPIST-1 over a three month period suggests that atmospheric escape has played an important role in the evolution of its seven planets—a process that’s still ongoing. Calculations made by Bourrier’s team suggests that the two innermost planets, TRAPPIST-1b and TRAPPIST1-c, have lost gigantic loads of water over the course of their history. These two planets, which receive the highest amount of ultraviolet energy, could have bled 20 Earth-oceans worth of water into space over the past eight billion years.

The TRAPPIST-1 HZ planets are much closer to each other than the Earth is to Mars, prompting the authors of this paper to wonder — if life could survive in the TRAPPIST-1 system, what’s the likelihood of life spreading from one planet to the next?

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